Electrical service providers such as electrical utilities employ electricity meters to monitor energy consumption by customers (or other entities). Electricity meters track the amount of energy consumed by a load (e.g. the customer), typically measured in kilowatt-hours (“kwh”), at each customer's facility. The service provider uses the consumption information primarily for billing, but also for resource allocation planning and other purposes.
Electrical power is transmitted and delivered to load/customer in many forms. For example, electrical power may be delivered as polyphase wye-connected or delta-connected power or as single phase power. Such various forms are known as service types. Different standard electricity meter types, known as meter forms, are used to measure the power consumption for the various service types. The commonly used meter forms in the United States include those designated as 2S, 3S, 5S, 45S, 6S, 36S, 9S, 16S, 12S and 25S meter forms, which are well known in the art.
FIG. 5 shows an exemplary split phase residential electrical service including a service transformer 202, an electricity meter 210, and a service entrance 270. The service transformer 202 includes a single-phase primary winding (not shown) and a center-tapped secondary winding 204. The secondary winding 204 includes a phase A portion 204A, a phase C portion 204C, and a neutral center tap 206 (which is connected to earth ground). The line-to-line voltage provided across the secondary winding 204 is typically 240V. The neutral center tap 206 on the secondary winding 204 results in a split of the 240V line-to-line voltage into a first line voltage of 120V (i.e., the line-side phase A voltage) and a second line voltage of 120V (i.e., the line-side phase C voltage), with the line-side phase A voltage 180° out of phase with the line-side phase C voltage (which line-side phase A and line-side phase C voltages may also be referred to herein as “phase A” and “phase C” voltages, “A phase” and “C phase” voltages, or alternatively, the “phase” and “anti-phase” voltages). With this arrangement, certain outlets within a residence may be configured to provide a lower voltage and other outlets within the residence may be configured to provide a higher voltage. For example, certain outlets may be configured to provide 120V for lighting and small appliances (i.e., these outlets connected between the phase A and neutral or phase C and neutral). Other outlets may be configured to provide 240V for large appliances (i.e., these outlets are connected across the two line conductors, thus providing the full line-to-line voltage). Because of the ground connection at the center tap 106 of the secondary winding, the circuits provided by the service transformer 202 are typically well balanced within a volt or two. Therefore, if a digital multi-meter is used to measure between neutral and phase A and phase C, both measurements will be close to 120V and 180 degrees out of phase.
The phase A and phase C voltages from the service transformer 202 are delivered to an electricity meter 210 before being passed on to the service entrance 270. The electricity meter 210 includes a number of circuits including a line-side sense circuit 220, a current transformer circuit 230, a measurement and control circuit 240, a disconnect switch circuit 250, and a load-side sense circuit 260. Each of these circuits may be provided with solid-state components or integrated circuits, as will be recognized by those of skill in the art.
The line-side sense circuit 220 is used to measure the line voltage (e.g., the phase A line voltage) delivered to the meter 210. The line-side sense circuit 220 includes two resistors, R1 and R2, which act as a voltage divider (. The value of resistor R1 is significantly greater than that of R2. In particular, in FIG. 5, the value of R1 is about 1000 times greater than that of R2. Accordingly, the output of the voltage divider is a small fraction of the actual phase A line voltage. The output of the voltage divider is fed to the “VA” input of the measurement and control circuit 240. The reference voltage for the voltage divider is provided at node 222, which is the phase C voltage. Accordingly, the meter 210 uses the phase C voltage as a hard reference for the voltage measurement.
The current transformer circuit 230 is used to measure the current (e.g., the phase A and/or phase C current) delivered to the meter 210. The current transformer circuit 230 includes a phase A current transformer 230A and a phase C current transformer 230C. The phase A current transformer includes a phase A conductor P1 and a secondary winding Si. The phase A conductor P1 is connected to the line-side phase A service transformer 204A, and the secondary winding Si is connected to the Ia input on the measurement and control circuit 240. Accordingly, the measurement and control circuit 240 is provided with an indication of the phase A current at the Ia input. Similarly, the phase C conductor P2 is connected to the line-side phase C service transformer 204C. Although not shown in FIG. 5, in at least one embodiment a secondary winding associated with the phase C conductor P2 is connected to the IC input on the measurement and control circuit 240. Accordingly, the measurement and control circuit 240 is further provided with an indication of the phase C current at the IC input.
The disconnect switch circuit 250 is used to disconnect the phase A and phase C voltages from the service entrance. The disconnect switch circuit 250 includes a phase A disconnect switch 250A and a phase C disconnect switch 250C. The phase A disconnect switch 250A is connected to the phase A conductor P1 and the phase C disconnect switch 250C is connected to the phase C conductor P2. The phase A disconnect switch 250A and the phase C disconnect switch 250 C are each controlled by the switch control output SC of the measurement and control circuit 240. The outputs of the phase A disconnect switch 250A and the phase C disconnect switch 250C lead to the service entrance 270. In particular, the output of the phase A disconnect switch 250A leads to the load-side phase A service entrance 270A, and the output of the phase C disconnect switch 250C leads to the load-side phase C service entrance 270C. When the phase A disconnect switch 250A and the phase C disconnect switch 250C are both open, no electrical power is provided to the customer at the service entrance 270.
The load-side sense circuit 260 is configured to detect the existence or non-existence of a customer load. The load-side sense circuit 260 includes a phase A resistive network 260A and a phase C resistive network 260C. The phase A resistive network 260A includes two resistors arranged as a voltage divider, including resistors R5 and R6. The reference voltage for the voltage divider of the phase A resistive network 260A is the Vref voltage provided at node 222, which is the phase C line voltage in the embodiment of FIG. 5. As shown in FIG. 5, the value of resistor R5 is similar to that of resistor R2, and the value of resistor R6 is significantly greater than the value of resistor R5. In particular, the value of resistor R6 is about 1000 times greater than the value of resistor R5. Accordingly, the output of the voltage divider is a small fraction of the actual phase A load voltage. The output of the phase A resistive network 260A (i.e., a phase load output voltage) is connected to the “LOAD_SENSE 1” input of the measurement and control circuit 240. Accordingly, the measurement and control circuit 240 is configured to measure an existing load associated with the phase A service entrance 270A.
The phase C resistive network 260C is similar to the phase A resistive network, and includes resistors R7 and R8, with an output (i.e., an anti-phase load output voltage) that is connected to the “LOAD_SENSE2” input of the measurement and control circuit 240 (resistor R7 in the phase C resistive network provides the output impedance and may also be referred to herein as the output load resistor of the voltage divider). Accordingly, using the phase C resistive network 260C, the measurement and control circuit 240 is configured to measure an existing load associated with the phase C service entrance 270C.
As noted previously, the meter 210 uses the phase C voltage as a hard reference for the measurement of other voltages (i.e., note that the phase C voltage is delivered to the measurement and control circuit as the “Measurement Ref” input). However, as explained in further detail below, even if no load is present at the service entrance, this arrangement may result in a phantom voltage being detected at the service entrance, and therefore prevent the service disconnect switch from being closed.
With continued reference to FIG. 5, consider a situation wherein the disconnect switch 250 is open, and no load is present at the service entrance 270 on the customer side of the meter 210 (e.g. the main service breaker is open or there are no electrical appliances, devices, lights, etc. of any kind installed). In this situation with no customer load and the service switches 250A and 250C open, the meter technician may use a DMM to measure a the following:
Load-side Meter A Phase to Neutral: 110 to 115 Vrms;
Load-side Meter C Phase to Neutral: 110 to 115 Vrms; and
Load-side Meter A Phase to Load-side Meter C phase: 0 Vrms.
These measurements assume 240 Vrms provided on the primary winding of the service transformer 202, and a digital multi-meter (DMM) input impedance of 10 M ohms. In this case, the neutral bypasses the meter and ties the customer service directly to the service transformer 202. Consequently, the neutral is unavailable for measurement purposes, and all voltage measurements are referenced to one line of the service (i.e., the C-phase line voltage in FIG. 5).
As shown in FIG. 5 in order to make the measurement of the Load-side Meter C phase to Neutral, the technician connects a DMM 280 between the Load-side C Phase and the neutral 272 at the service entrance 270. However, in the absence of a customer side load, when the DMM 280 is placed across the Load-side Phase C service entrance 270C and the neutral 272, a circuit is completed which resulting in a phantom Voltage being displayed on the DMM. In other words, as shown in FIG. 5, if a DMM 280 is connected between the Load-side C Phase 270C and the neutral 272 at the service entrance 270, a complete circuit is formed that extends from earth ground at neutral 272, across the DMM 280, across the Load-side C Phase service entrance 270C, across R6 and R7 of the resistive network 260C of the load-side sense circuit 260, across the reference voltage (Vref) and the phase C portion 204C of the service transformer 202, and returning to earth ground at neutral center tap 206. Because of this completed circuit, the DMM 280 will register a voltage measurement, which appears to the utility as a load resistance, although this is actually a phantom load since no load is actually present at the service entrance 270. The phantom voltage may be around 110 to 115 Vrms due to the voltage divider formed by the input impedance of the DMM (e.g., 10 M ohms) and output impedance of the load-side sense circuitry 260 when no customer load is present. While the DMM 280 only detects a small current flow in this arrangement, this small current flow is sufficient to indicate the existence of some load at the service entrance.
In the event the disconnect switches 250A and 250C are open, and a load is detected at the service entrance 270, the detected load will prevent the disconnect switches 250A and 250C from being closed (whether by the technician or other means). This is true even if the load is a phantom load, as described above. In particular, even though it would be safe to close the service disconnect switch with a phantom load (since no actual load is present), it may not be safe to close the service disconnect switch with a real load condition (which may indicate co-generation by the customer or another third party tampering condition). Unfortunately, a phantom load cannot be differentiated from a real load condition based simply on readings from the measurement and control circuit 240 in a traditional utility meter. Accordingly, in these situations, the utility must investigate the actual conditions of the detected load (whether phantom or real) prior to closing the service disconnect switch.
In view of the foregoing, it would be advantageous to provide an arrangement for an electricity meter that avoids the detection of a phantom load when no load is actually present on the customer side of the meter at the service entrance. It would be further advantageous if such arrangement for an electricity meter could predict the existence of any unsafe conditions at the service entrance prior to closure of the disconnect switches 250A and 250C.